In conventional electrophotographic processes a latent charge image is formed on a recording surface and is then developed into a visible image by applying a pigmented developer material. A recording surface may consist, for example, of a photoconductive layer which is initially provided with a uniform electrostatic charge. The photoconductive layer is then selectively discharged in an image wise manner by exposing the recording layer to a light pattern corresponding to the image to be reproduced. This produces a latent electrostatic image to which charged developer particles will adhere. A developed image can be fixed or rendered permanent in various ways, such as applying heat, pressure, solvents and any combination of the above.
The foregoing is essentially an optical image reproduction process and is employed in most types of commercially available document photocopying machines and laser type printers. The photoconductive layer may be provided on the final recording medium or more often it may be provided on the surface of an intermediate transfer member such as rotary drum.
Various methods have been employed for developing the latent charge images created by the electrophotographic technique. One early developing method involved cascading the developer material across the latent image areas to be developed. Another method, referred to as powder cloud development, involved dispersing the developer particles in a moving stream or flow of air and then bringing the entraining particles into contact with the latent image bearing surface. Rotating fur brushes were also used to apply the developer particles to the recording surface in some early types of electrophotographic imaging apparatus.
A more common developing method at the present time is referred to as magnetic brush development. This involves the use of a magnetic element, typically in the form of a cylindrical roll, for carrying the developer material and applying it to the latent image bearing surface. The developer material may be of the two component type in which finely divided and pigmented toner powder is interspersed with somewhat large ferromagnetic carrier particles. Alternatively, the developer material may be of the single component type in which only one kind of particle is involved. A common type of single type component developer consists of fine particles of magnetic material, such as iron or iron oxide, encapsulated within a resin having a relatively low softening temperature. A suitable pigment such as carbon black is usually added to the resin in order to impart the desired color to the developer material.
When placed in a magnetic field a developer material of either the two component or single component type will form streamers resembling the bristles of a brush, similar to the way in which iron filings will align themselves with the magnetic flux lines at the ends of a bar magnet. This property is exploited in magnetic brush developing systems by utilizing a magnetic roll assembly to retain a brush like layer of developer material on its peripheral surface. The layer of developer is brought into the proximity of the latent image bearing surface which is usually moving in a direction normal to the roll axis as the roll itself rotates. The brushing action brings a developer material into intimate contact with the recording surface and permits electrostatic transfer to the developer particles from the roll to the latent image areas.
A number of different structural configurations have been employed in magnetic brush developing systems. The simplest arrangement is an exposed magnetic roll which carries a layer of developer material on its peripheral surface. The roll may be magnetized in various ways even intrinsically or by covering the peripheral surface of the roll with magnetic material. An alternative arrangement more widely used at the present time is a two part roll assembly consisting of an inner magnet magnetic element enclosed within an outer non-magnetic shell or sleeve. The shell is usually cylindrical in shape and provides a smooth carrier surface over which the developer particles can slide while being held by the inner magnetic element. The magnetic flux density at the shell surface is a function of the spacing between the shell and the inner magnetic element and of the magnetic permeability of the shell material. Therefore by appropriate selection of these factors it is possible to obtain close control over the magnetic field strength that is used to hold the developer particles on the surface of the shell. Another advantage of the shell is that it provides a useful barrier against contamination of the inner magnetic element and associated bearing, shafts, and the like with developer particles.
Various types of two part magnetic brush rolls have been proposed. In one form of the device the inner magnetic element rotates while the outer shell is held stationary. The rotation of the inner magnetic element causes a backward tumbling or somersaulting motion of the developer particles on the outside circumference of the shell resulting in a net propagation of the developer material in the direction opposite to the rotational direction of the magnetic element. The propagation rate of the developer particles is much less than the rotational speed of the magnetic element but is sufficient to assure continuous flow of developer particles into the developing zone. In another form of the device, the outer shell itself rotates with respect to the inner magnetic element which is held stationary. This embodiment is usually used with two component developers since the rotation of the outer shell induces thorough mixing between the toner and carrier particles and continues replacement of spent developer at the developing zone.
In embodiments where a rotatable shell is employed it is possible to control the rate of movement of the developer material by varying the rotational speed of the shell hence it is possible to deliver more developer material to the developing zone by increasing the rotational speed of the shell and conversely less developer material is carried to the developing zone when the shell speed is reduced. In the fixed shell embodiments a similar but less pronounced effect can be obtained by varying the speed of the inner magnetic element.
Although the foregoing discussion has been concerned primarily with the development of latent electrostatic images formed by the electrophotographic process, it should be pointed out that similar considerations apply to both magnetic imaging systems and electrostatic printing techniques as well. With electrostatic printing techniques a latent charge image is created non-optically on a diaelectrically charged retentive surface by means of an electrostatic printhead which is typically of a dot matrix type. The dialectic layer, like the photoconductive layer in electrophotographic apparatus, may be provided either on the final recording media or on an intermediate transfer member. Magnetic imaging can be carried out by magnetizing the selected areas of a layer of magnetic material using a magnetic recording head. Alternatively, magnetic imaging can also be accomplished by imparting uniform magnetization to a layer of magnetic material and then selectively demagnetizing the material in an image wise pattern by raising the temperature of the selected areas above the Curie Point of the material. Either method leaves a layer of material with a latent magnetic image which can be rendered visible by the application of a magnetically attractable developer material.
With rotatable shell embodiments it is important that a sufficient supply of developer material reside behind a rotatable shell. Typically this is accomplished by providing a large developer well behind the shell. While this design is simple, because the developed material is located equally along the length of the developer, it does require a considerable amount of space. Additionally, if the toner is refillable, it may be difficult to add toner evenly along the developer.
Present trends for electrophotographic printers are forcing a significant reduction in size. The aftermentioned large reservoir behind the developer consumes a considerable amount of space which could be put to better use, One size reduction approach relocates the reservoir to one side of the developer.
To allow the developer material to be stored at one end of the developer some apparatus use an auger to deliver the developer material down the length of the developer. The auger is an extended auger with a rotating helical element that drives the toner in a similar manner to a grain auger. Adding an auger to an existing developer requires additional parts and costs along with decreased mean time between failure. Therefore it is desirable to integrate the developer function such that an additional auger for delivery of the developer material is not necessary.